1. Field of the Invention
The invention relates to mobile cranes, and more particularly, to mobile cranes having main and auxiliary counterweight assemblies.
2. Discussion of the Related Art
Mobile cranes are well known for lifting heavy loads ranging from a few tons to hundreds of tons. Such cranes typically include a counterweight assembly which prevents or at least inhibits excessive boom deflection which could otherwise occur when the crane lifts heavy loads. Cranes of this general type are disclosed, for example, in U.S. Pat. Nos. 3,842,984 to Brown; 4,258,852 to Juergens; 4,540,097 to Wadsworth; and 5,035,337 to Juergens.
Cranes of the above-mentioned type typically include 1) an earthborne base, usually mounted on wheels or crawler treads, and 2) a platform mounted on the base for rotation about a vertical slewing axis. A boom is pivotally connected to the platform for swinging substantially in a vertical plane that contains the slewing axis. The boom is inclined with respect to the slewing axis such that it extends upwardly and forwardly from the base. A load line passes over the upper end of the boom and has one end which depends from the boom to be connectable with the load and an opposite end connected to a winch on the platform. The platform typically has a tail-like rearwardly-projecting portion to which is attached a main counterweight assembly that offsets the forward tilting forces exerted by the boom and by any light to moderately heavy load hoisted by the crane.
Cranes of the type described above may be provided with an auxiliary counterweight assembly to offset tilting effects imposed by extremely heaving loads. For instance, the above-mentioned Juergens '337 patent discloses a crane having 1) a conventional boom, 2) a conventional tail-like projection and the associated main counterweight assembly, and 3) a mast pivotally connected to the platform at a location behind the boom connection so as to allow the mast to swing relative to the platform in substantially the same plane that contains the swinging of the boom and the slewing axis. A tension line is connected between the upper ends of the boom and the mast so as to cause the boom and the mast to swing in unison and to normally cause the mast to project upwardly from the platform at a rearward inclination to the vertical slewing axis. An auxiliary counterweight assembly is attached to the top of the mast and is normally ground-supported. When a heavy load is being hoisted, the boom tends to swing forwardly and downwardly in reaction to the lifting forces exerted by the load, causing the entire mast to swing correspondingly forwardly and upwardly. Mast swinging lifts the auxiliary counterweight assembly off the ground so that the full weight of the auxiliary counterweight assembly, in addition to the weight of the main counterweight assembly mounted on the tail-like projection of the platform, is operable to offset the tilting forces exerted upon the boom by the load. In addition, a gantry structure, fixed on the rear part of the platform, is provided and has a lost motion connection with the mast that defines the forward limit of swinging motion of the boom relative to the platform.
Cranes of the type described above, though satisfactory, exhibit several drawbacks and disadvantages.
For instance, for transport from one job site to another, the crane must be partially disassembled into units that comply with size and weight limitations prescribed for highway vehicles. This requires that mobile cranes be as compact, light, and easy to assemble and disassemble as is consistent with its hoisting capacity. A crane of this type should also have the smallest possible tail swing, that is, the upper structure, including counterweight assemblies, should project the least possible distance behind the vertical slewing axis so that it can operate in a relatively restricted space without interference from obstacles around its rear. All of these goals are hindered by providing a relatively long spar or tail mast of the type employed by most cranes. Some, but not all of these problems are addressed and at least partially solved in the Juergens '337 patent.
Moreover, even in systems such as that disclosed in the Juergens '337 patent having an auxiliary counterweight assembly, the entire spar or mast pivots about the platform upon main boom deflection. As a result, the main and auxiliary counterweight assemblies come into play simultaneously such that substantially all boom deflection occurs against the total reactive forces of the main and auxiliary counterweight assemblies and against the weight of the entire spar in all instances. Therefore, the number of auxiliary counterweights need to be precisely determined and controlled so as to provide the required reaction forces. The total weight of the spar and the auxiliary counterweight assembly also must travel forward in a dynamic, uncontrolled manner because the spar travels upward, pivoting at the base of the platform. Consequently, the center of gravity moves forward along with the load of the boom, thereby actually accelerating boom swinging--a result which is exactly opposite that sought through the use of the auxiliary counterweight assembly.
Some of the above discussed problems are exasperated in the case of multi-section, pendant-supported, telescopic booms of the type having a pendant pay-out system for preventing the boom from bending downwardly while lifting heavy loads. The conventional system disclosed, for example, in U.S. Pat. No. 4,492,312 to Poock, employs an external pendant pay-out system to straighten the boom and to add additional counterweights to compensate for heavy loads. Such external pendant pay-out systems are used in addition to the spar or mast supported auxiliary counterweight assemblies of the type described above, thereby complicating the system.